In a conventional gas turbine engine and the like, operational efficiency generally increases as the temperature of the combustion stream increases. Higher combustion stream temperatures, however, may result in the production of higher levels of nitrogen oxides (“NOx”) and other types of undesirable emissions. Such emissions may be subject to both federal and state regulation in the United States and also may be subject to similar regulations abroad. A balancing act thus exists between operating the gas turbine engine within an efficient temperature range while also ensuring that the output of nitrogen oxides and other types of regulated emissions remain below the mandated levels.
Several types of known gas turbine engine designs, such as those using Dry Low NOx (“DLN”) combustors, generally premix the fuel flows and the air flows upstream of a reaction or a combustion zone so as to reduce nitrogen oxide emissions via a number of premixing fuel nozzles. Such premixing tends to reduce overall combustion temperatures and, hence, nitrogen oxide emissions and the like.
Premixing, however, may present several operational issues such as flame holding, flashback, auto-ignition, and the like. These issues may be a particular concern with the use of highly reactive fuels. For example, given an ignition source, a flame may be present in the head-end of a combustor upstream of the fuel nozzles with any significant fraction of hydrogen or other types of fuels. Any type of fuel rich pocket thus may sustain a flame and cause damage to the combustor.
Other premixing issues may be due to irregularities in the fuel flows and the air flows. For example, there are several flow obstructions that may disrupt the flow through an incoming pathway between a flow sleeve and a liner. With a combustor having fuel injector vanes that inject fuel into the airflow upstream of the head-end, these flow disturbances may create flow recirculation zones on the trailing edge of the vanes. These recirculation zones may lead to stable pockets of ignitable fuel-air mixtures that can in turn lead to flame holding or other types of combustion events given an ignition source.
One example of a flow obstruction is a crossfire tube. Generally described, a crossfire tube may be used to connect adjacent combustor cans. The crossfire tubes provide for the ignition of fuel in one combustion can from the ignited fuel in an adjacent combustion can. The crossfire tubes thus eliminate the need for a separate igniter in each can. The crossfire tubes also serve to equalize the pressure between adjacent combustor cans. The crossfire tubes generally are positioned upstream of the premixing fuel injectors and pass through the incoming flow path between the liner and the flow sleeve. As such, the crossfire tubes may cause a wake in the flow path that may envelop one or more of the premixing fuel injectors. As described above, such a wake may cause recirculation zones and, hence, fuel holding and other types of flow disturbances.
There is therefore a desire for an improved combustor design. Such an improved design should accommodate flow disturbances caused by crossfire tubes and the like so as to avoid flame holding, flashback, auto ignition, and other types of flow disturbances. Moreover, such an improvement should provide increased efficiency and extended component lifetime.